This invention relates to an improvement in a motion picture camera capable of continuous shooting, as well as one-frame shooting. More particularly, this invention concerns a motion picture camera, wherein an improved electric circuit for switching between the continuous shooting operation and the one-frame shooting operation is provided for better performance.
FIG. 1 shows the principal part of a typical prior art electric circuit for a motion picture camera capable of both continuous shooting and one-frame shooting. In FIG. 1, a changeover switch Sc is provided for switching between continuous shooting and one-frame shooting. This switch Sc is closed for one-frame shooting and is opened for continuous shooting. A release switch Sr is interlocked with the shutter-release button (not shown) of the camera so as to be closed during releasing of the shutter. An electromagnet L is provided for driving the shutter-stop lever which is to be engaged into an indent of a cam linking shutter blade, so as to stop rotation of the cam, when deenergized. Upon energization, the electromagnet L releases the cam and permits the shutter blade to be rotated by a motor. A transistor T is provided for controlling the electromagnet L. R10, R11, and R12 are resistors, and C is a capacitor. The resistance of the resistor R10 is selected to be far smaller than that of resistor R11. A terminal +E is to be connected to the positive end of a D.C. power source.
In the continuous shooting condition, when the release switch Sr is closed, the base of the transistor T receives a voltage which is obtained by dividing the source voltage by the drop in resistors R11 and R12, so that the collector current of the transistor T is increased to energize the electromagnet L and permit the shutter blade to rotate continuously. The resistor R11 is sufficiently large that capacitor C1 discharges over a very long period of time to permit continuous operation.
In the one-frame shooting, before closing the release switch Sr, the capacitor C1 has been charged to to the level of a voltage of the power source applied to the terminal +E. When the release switch Sr is closed, the voltage of the capacitor C is impressed to the base of the transistor T and turns it "on", and then the charge of the capacitor C1 is discharged through the resistors R11 and R10 at a certain short time until the voltage at the base of the transistor T is reduced to the level of the aforementioned divided voltage. Due to the decrease of the base voltage of the transistor T, the collector current of the transistor T is reduced and hence, the current of the electromagnet L decreases within the short time. Accordingly, the shutter-stop lever stops the shutter blade.
The lapse time from the close of the switch Sr to the finish of decrease of the collector current of the transistor T is dependent on a discharging time constant C1.R10, the effective impedance value of the parallel combination of R10 and R11 being substantially R10. The lapse time should be short enough to release the shutter-stop lever SSL from the indent of the cam Cm sufficiently before completion of each whole period of motion of the one-frame shooting. For attaining such short lapse time, the resistance of the resistor R10 should be small. On the other hand, in the one-frame shooting, it is necessary that the collector current of the transistor T decreases to a very small level, and therefore, the ratio R12/R10 should be sufficiently large. However, in repetition of one-frame shootings, it is necessary that the capacitor C is sufficiently charged up during every period of closing of the release switch Sr. For this reason the resistance of the resistor R12, which defines the time constant for charging the capacitor C1, should not be large. These are difficulties in designing the conventional circuit shown in FIG. 1.
Moreover, there is a problem due to instability of the transistor T. In the circuit of FIG. 1, the small amount of collector current of the transistor T remains even during the one-frame shooting operation. A residual collector-current, namely the current of the electromagnet L, is considerably influenced by the environmental temperature. Accordingly, the lapse time from the close of the release switch Sr to the deenergization of the electromagnet L is influenced by the temperature. That is to say, in cold temperatures, the lapse time becomes larger, causing the shutter-stop lever to release the shutter for too long a time. Such unexpected long release of the shutter causes defective two-frame shooting instead of the one-frame shooting. Furthermore, in the circuit of FIG. 1, the residual current flows through the electromagnet L and the transistor T even in the continuous shooting operation. The residual current becomes excessively large at high temperature and wastes the electric power of the battery.